A structural diagram of a top sealing plate used for a conventional cylindrical cell is shown in FIG. 8, FIG. 10 and FIG. 11. Also, a cell provided with a conventional top sealing plate is shown in FIG. 9 and FIG. 12. A secondary battery such as a nickel cadmium, nickel hydrogen, and lithium battery comprises a battery case 1, power generating elements, lead plate 8, and top sealing plate 52. The power generating elements include a positive electrode, negative electrode, separator, and electrolyte. These power generating elements and lead plate 8 are stored in the battery case 1. The top sealing plate 52 is disposed so as to seal the opening of the battery case 1 via a gasket 7. The gasket 7 also serves a function of electrical insulation. Further, when a large volume of gas is abnormally generated in the battery case, the top sealing plate 52 functions to discharge the gas out of the battery case 1.
The top sealing plate 52 includes a convex portion 6, filter 53, cap 51, and contacts of these parts. The convex portion 6 is projected at the center of the top sealing plate 52. The convex portion 6 serves as a positive output terminal as well. The filter 53 is installed at the bottom thereof. The lead plate 8 is connected to the positive electrode. The lead plate 8 is welded to the filter 53. It is required that the top sealing plate 52 itself be low in electric resistance.
The top sealing plate 52 is available in a variety of shapes. An example of conventional top sealing plate 52 is shown in FIG. 8. FIG. 10 shows a type of top sealing plate 52 which is different from the one shown in FIG. 8. FIG. 11 shows the component parts of the top sealing plate shown in FIG. 10, illustrating what they are just before being assembled.
A brief description of the top sealing plate shown in FIG. 8 will be given in the following. The main section of the top sealing plate 52 comprises a cap 51, filter 53, and valve body 12. Generally, the cap and filter 53 are manufactured by a process where a sheet steel of about 0.3 mm thick is repeatedly pressed several times, followed by a process where the sheet steel is nickel-plated. The valve body 12 is a rubber part having a cylindrical or prism-like shape. The valve body 12 closes the valve hole 9. When the gas pressure in the battery case 1 is abnormally increased, the valve body is deformed, and then, the valve hole 9 disposed at the center of the filter 53 is opened. The gas is discharged out of the battery case 1 through gas vent hole 10. Thus, the valve body 12 has a function of discharging abnormal gas out of the battery case 1.
The convex portion 6 formed at the center of the cap 51 is used as a positive terminal. The inside of the convex portion 6 has a function of storing the valve body 12, a function of positioning the valve body 12 to the valve hole 9, and a positioning function that enables pressing the valve body against the valve hole with a specified force. At the outer periphery of the cap 51 is disposed a flange portion so as to nearly uniformly surround the entire periphery of the convex portion 6. Several gas vent holes 10 of about 1 mm in diameter are formed in the flange.
The central portion of the filter 53 has a rather deeply recessed dish shape. There is provided a valve hole 9 of about 1.5 mm in diameter at the central portion of the filter 53. The valve hole 9 serves a function of discharging the gas. With the valve body 12 stored inside the convex 6 of the cap 51, the cap 51 is fitted to the filter 53 and is centered thereto, and after that, the opening end 63 of filter 53 is inwardly pressed for caulking purpose. Finishing these processes, the cap 51 and the valve body 12 are secured to each other by the filter 53. In this way, the valve hole 9 is closed with the valve body 12. Thus, the top sealing plate 52 is completed.
However, as shown in FIG. 8 and FIG. 10, a conventional top sealing plate commonly employed is based upon the structure and manufacturing method as described in the following. That is, pressing and caulking forces are applied to the bend portion of the filter, during which time the opening end of the filter is firmly holding the flange portion of the cap under a contact pressure. However, after releasing the pressing force, the caulking strength between the flange portion of the cap and the bend portion of the filter is not high enough and unstable due to springback of the bend portion of the filter. Further, the contact pressure and contact area are very slight, and therefore, the electric contact resistance between the flange portion of the cap and the bend portion of the filter is very low and unstable. Particularly, there exist such problems in a top sealing plate used for UM-4 size cells or smaller whose outer diameter of cylindrical cell is less than φ8 mm.
Also, the caulked portion of the top sealing plate is subjected to temperature cycles and thermal shocks during use of the battery. Due to such thermal shocks and secular change, the contact pressure between the filter and the cap will be lowered. Further, the state of electrical conduction becomes unstable due to film oxide generated on the contact surfaces. Accordingly, the battery increases in internal resistance, and the output voltage of the battery is lowered.
A method of solving such problems is disclosed in Japanese Laid-open Patent No. 10-159074. The teachings of Japanese Laid-open Patent No. 10-159074 include a method comprising a process of fitting a cap 51 to a dish-shape top sealing plate having a function as a filter, and a subsequent process of inwardly bending the opening end of the dish-shape top sealing plate and securing the cap and valve body 4, and further, a process of welding the bend portion of dish-shape top sealing plate and the cap, thereby making perfect the state of electrical conduction of top sealing plate 11.
Due to the above method comprising a process of welding the bend portion of dish-shape top sealing plate and the cap, it can be anticipated that the electric resistance at the joint between the filter and the cap will be lowered, enhancing the reliability, and that the state of electrical conduction obtained will be satisfactory. However, a processing method by welding is very poorer in productivity, and also, higher in manufacturing cost as compared with a processing method by pressing. Further, in a processing method by welding, the product value will be lowered in terms of rust preventing measure and appearance. Thus, a processing method by welding is not satisfactory enough judging from a general point of view.